1. Field of the Invention
The present invention relates to fluid delivery systems, and more particularly to multiple port fluid delivery systems for use in automated chemistry processing instruments.
2. Description Of Related Art
For fluid delivery systems designed for handling several types of fluids in a flow system, one of the design concerns is to reduce cross contamination between the fluids. Especially for systems handling chemical reagents, cross contamination between different reagents often adversely affects the chemical integrity of the reagents and thus the efficiency of the controlled chemical reactions that involve such reagents. For example, in an automated nucleic acid synthesis instrument, various steps are carried out by a reagent delivery system which dispenses a number of chemical reagents in a predetermined sequence in a cycle into a synthesis reaction column, in accordance with instructions from the system controller or computer. The synthesis efficiency depends in part on the integrity of the reagents.
As a background, the chemistry of nucleic acid synthesis (generally referred to as "DNA synthesis") is well known. Generally, this is the process of constructing synthetic single-stranded oligonucleotide through linking of various nucleotides which are the basic building blocks of DNA. DNA synthesis is described generally in U.S. Pat. No. 4,458,066 issued to Caruthers et al, entitled "Process for Preparing Polynucleotides", which is incorporated by reference herein. The process described therein constructs a single-stranded oligonucleotide using one of several approaches in synthesizing DNA, namely the so-called solid-phase phosphoramidite method which generally involves the steps of deblocking/activation, coupling, capping and oxidation in each synthesis cycle for linking a building block on a solid-phase support. Further reference to this process of DNA synthesis may be found in "Oligonucleotide Synthesis - A Practical Approach" edited by M. J. Gait, IRL Press, 1984, which is incorporated by reference herein; and in particular Chapter 3 therein entitled "Solid-Phase Synthesis of Oligodeoxyribonucleotide by the Phosphite-Triester Method" written by Tom Atkinson and Michael Smith. It is suffice to understand for purpose herein that the reagents are delivered to the reaction column via several valves.
The effectiveness of the DNA synthesis process is very sensitive to the purity of the reagents. Cross contamination between the reagents adversely affects the production of oligonucleotide. One source of cross contamination is in the valves, particularly multi-port valves which select delivery between different reagents. There are inevitable dead volumes in the valves associated with switching between reagents. For this reason, past instrument designs which have required absolute control over cross-port contamination and random selection of chemical reagents have avoided use of multi-port valves because of the difficulties in preventing such contamination. However, the adopted design of the fluid delivery system is relatively complex and requires additional effort in designing the control scheme for the system.